1. Field of the Invention
The present invention relates to a resin composition containing a metal salt. This resin composition has long lasting antistatic properties and a low surface resistivity.
2. Description of the Prior Art
Due to the advangtages of good mechanical properties such as impact resistance, polymer materials have been V utilized extensively. Plastic materials can be processed into various kinds of plastic articles using variety processing methods such as extrusion, calendering, injection, and compression. However, due to the insulation characteristics of polymer materials, static electricity easily accumulates on the plastic articles made therefrom. Slight static electricity makes people feel uncomfortable, while severe static electricity can even cause industrial damage.
In recent years, electronic devices follow the trend of high-density and high-speed. Therefore, they are more sensitive to the static electricity than before. A static charge with a potential of 50 to 300 V (volt) can cause severe damage to electronic devices. Since a human body carries an average of 1 to 2 kV (kilovolt) of static electricity, when a person touches the electronic devices, the static electricity carried by the human body can bring functional damage and memory loss to the electronic devices. Therefore, during manufacturing or transporting, the electronic devices should be put in containers made of antistatic materials to dissipate static electric charges.
Several solutions have been proposed to prevent the electrostatic damage. The first solution is to coat a layer of antistatic agent on the plastic surface by, for example, a dipping, wiping, or spraying process. Such an antistatic agent is usually a moisture-absorbing surfactant. One drawback to this kind of antistatic agent is that the antistatic effect decreases as the ambient humidity decreases. Moreover, the coating of this antistatic agent can easily be peeled off from the plastic surfaces, thus contaminating the electronic devices. Once the antistatic coating is peeled off, the plastic material should be treated again to form a new coating, which is very inconvenient.
The second solution to overcome the electrostatic damage is to incorporate a migration type antistatic agent into the polymer matrix during the manufacture of the plastic material. By means of the migration of such low molecular weight antistatic agent to the surface of plastic material, the antistatic agent will absorb water molecules in the atmosphere to form a water molecule layer. The static electric charges can thus be dissipated. Examples of this kind of antistatic agents include N-oxyethylated alkylamines disclosed in U.S. Pat. No. 3,575,903, alkanolamines and polyalkylene glycols disclosed in U.S. Pat. No. 3,625,915, and lauric diethanolamide disclosed in U.S. Pat. No. 3,873,645. The drawback of this type of low molecular weight antistatic agent is that it will migrate and adhere to the plastic surface, or even bloom on the surface of the plastic article. Moreover, the antistatic properties will gradually decrease after a long period of use.
The third solution to overcome the electrostatic damage is to incorporate a high molecular weight antistatic agent during the manufacture of plastic materials. Such an antistatic agent generally imparts permanent (long lasting) antistatic properties to the plastic material and can be classified into two types in terms of its compatibility with the polymer matrix. The first type of antistatic agent has a limited compatibility with the polymer matrix. Therefore, a highly dense continuous phase of the antistatic agent will be formed on the surface of the plastic article. Such an antistatic agent phase distributed on the surface can thus provide antistatic effect. However, in order to make this antistatic agent distribute appropriately, a compatibilizer is generally added in the system. In case no compatibilizer is added or an unsuitable compatibilier is added, the properties of the polymer matrix will be adversely affected. Examples of this kind of antistatic agents include the polyether ester amide disclosed in Japanese Pat. Publication No. 4-337344 and U.S. Pat. No. 5,338,795, and the epichlorohydrin copolymer disclosed in U.S. Pat. Nos. 4,588,733, 4,775,716, 4,857,590, and 5,216,046. The second type of antistatic agent, which is obtained by copolymerizing an alkylene oxide-containing vinyl monomer with an ordinary vinyl monomer, has a better compatibility with the polymer matrix. When such an antistatic agent is blended with a polymer matrix, the vinyl group can provide compatibility, and the alkylene oxide group can provide antistatic properties. Since this antistatic agent has suitable compatibility with the polymer matrix, it is distributed evenly in the matrix after blending. This can impart the plastic with long lasting antistatic properties and improved mechanical properties. This kind of antistatic agent can be found in U.S. Pat. No. 4,543,390, Japanese Patent Publication No. 60-144314, No. 60-195143, No. 61-261344, No. 62-48756, and No. 1-126358. However, the antistatic properties still need to be improved.
Adding other additives into a polymer matrix is another way to improve antistatic properties. In U.S. Pat. No. 4,872,910, LiCl and a polyethylene oxide (PEO) are added in an acrylic latex. However, since the PEO is introduced by means of addition, the compatibility is not satisfied. In U.S. Pat. No. 5,683,862, a lithium salt and an ethylene oxide/propylene oxide (EO/PO) copolymer are added in an acrylic latex. The EO/PO copolymer is also introduced by means of addition, the compatibility is not satisfied either. In U.S. Pat. No. 5,283,008, a metal oxide is added in a poly(alkylene oxide)vinyl carboxylic ester. In U.S. Pat. No. 5,998,546, an antistatic resin composition is disclosed, which includes a hydrophilic copolymer and a thermoplastic resin. The hydrophilic copolymer includes a hydroxide of an alkali metal and a monomer mixture.